1. Field of the Invention
The present invention relates to tubular connections of the kind commonly used in the oil industry. In particular, the tubular connection of the present invention comprises a stabilized center-shoulder seal, wherein stabilization is provided by specialized thread constructions adjacent the center-shoulder seal. The invention also pertains to a specialized center-shoulder seal design. Further, the invention pertains to a thread profile which provides a self-centering function upon makeup of the tubular connection. This thread profile can be used with the stabilized center-shoulder seal connection as well as other connection designs known in the art.
2. Background Art
The present invention is applicable broadly for use in joining pipes, couplings and the like, and has particularly been developed for use in oil industry tubular connections, since these connections experience more severe and demanding conditions than those observed for other industries and applications. In particular, the trend in the oilfield is to minimize pipe diameter and to conserve borehole diameter. The smaller the diameter of the borehole for the well, the greater the conservation of energy and reduction of drilling costs. However, in utilizing smaller diameter pipe, it is important that the connections between adjacent tubular pipes be transparent in geometry and performance characteristics, i.e. the geometry and performance characteristics of the connection be substantially the same as that of the tubular pipe itself. Preferably, the pipe string should behave as if the pipe connections were not there. The ideal connection would have the same geometry as the pipe, such as the same inside and outside diameters, and would have the same performance characteristics such as tension rating, compression rating, internal and external pressure ratings, torsion resistance, and bending resistance, for example.
Two types of oilfield connections, namely flush joints and slim line connections, have been utilized to conserve hole diameter. The outer diameter of a flush joint connection is generally the same as the outside diameter of the tubular pipe. The outside diameter of a slim line connection is generally 2 to 3.5% greater than that of the pipe. Although the objective of such prior art pipe has been to achieve total geometric and performance characteristic transparency, such has not been possible. Often, the performance characteristics of prior art pipe have been sacrificed to achieve greater geometric transparency.
Various standards have been set for evaluating the efficiency of a pipe connection. One such efficiency is determined by the load capacity of the pipe and pipe connection in the axial direction. This tension efficiency is calculated by comparing the fracture strength of the connection with the fracture strength of the tubular pipe. Most prior art flush joint and slim line connections achieve an efficiency in the range of 65% to 75%, respectively. This efficiency, of course, varies with the type of connection, the pipe diameter and thickness, and the method of manufacture of the connection.
Prior art connections include the basic elements of threads, shoulders, and seals. The following prior art patents describe various features which have been used to produce flush and slim-line connections.
U.S. Pat. No. 671,274 to W. C. Fischer, issued Apr. 2, 1901, shows a dovetail-shaped thread design which can be applied to the outside of one tubular and the inside of another tubular to be connected together.
U.S. Pat. No. 1,474,375 to J. J. Moore, issued Sep. 20, 1923 shows what applicant has described as single hook thread design. The thread is used to fasten the forward end of the after-body of a torpedo to an air flask body.
U.S. Pat. No. 1,927,656, issued Sep. 19, 1933 to G. M. Eaton et al., describes a pipe joint for fastening together the consecutive pipe lengths of a pipe string in a manner to obtain strength at the joint without increasing the wall thickness (or with a minimum increase thereof) in the portion of the pipe in which the screw threads are cut. One of the two pipes to be joined is machined on the outside surface of the pin and the other is machined on the inside surface of the box so the two pipes can be screwed together to provide what would today be called a slim line connection. The threaded portion of each pipe is machined on a taper with the depth of the turns of the threads being the greatest at the center of the threaded portion and decreasing gradually in both directions to vanishing points at the ends of the threaded portions.
U.S. Pat. No. 2,062,407, issued Dec. 1, 1936 to G. M. Eaton et al., discloses a preferred form of pipe joint wherein the height of the threads in the threaded portion of the joint is at a maximum in the central area of the threaded portion of the joint and decreases gradually in both directions to vanishing points, somewhat similar to the joint described above. However, the thread design differs in that the vanishing of the threads corresponds with the intersection of thread fabrication lines which are not only based on a taper relative to the pipe longitudinal axis but also a line which is cylindrical or parallel to the pipe longitudinal axis. Specifically, the root of the thread at the pipe exterior edge position of the pin is located along a line parallel with the pipe longitudinal axis while the crest of the thread at the same location is located along a line at a constant taper with the pipe longitudinal axis. The root of the thread at a position more interior from the edge of the pipe is located along a line at a constant taper with the pipe longitudinal axis, while the crest of the thread at the same location is located on a line of parallel taper to the root for a distance. Finally, at the most interior thread location on the pin, the thread root continues on a taper with the pipe longitudinal axis while the crest of the thread is constructed along a line parallel with the pipe longitudinal axis. Thus, at both the exterior and interior edges of the threaded connection, the thread disappears where thread root and thread crest intersect. The thread construction of the box is complimentary to that of the pin so the two threaded members can be interengaged.
U.S. Pat. No. 2,207,005 to R. D. Haas, issued Jul. 9, 1940, pertains to a single hook thread form (as defined herein) wherein the pin member load flank is undercut to provide a cam action under pressure to draw the connection members together and establish a sealing-off function. The thread form is constructed on a taper to the longitudinal axis of the connection.
U.S. Pat. No. 2,488,566 to R. S. Sperry, issued Nov. 22, 1949 described a tapered thread structure having a lead on one flank of the helical thread which differs from the lead on the other flank of the helical thread, to provide a constantly increasing thread width from one end of the thread structure to the opposite end. This variable width thread structure is said to enable easy disassembly of a rod or pin from a socket, permitting separation of pin from socket without the need to completely unscrew the parts, while providing a wedge-like, snug interfit between the threads of the parts when coupled together. The preferred embodiment for use of the variable width thread structure is on the surface of a pull rod used in combination with a tubular work piece to be formed.
U.S. Pat. No. 3,109,672, issued Nov. 5, 1963 to William F. Franz describes a threaded pipe joint comprising a pipe member having a cylindrical outer surface and a tapered buttress thread at the end thereof vanishing along the outer cylindrical surface, providing a length of fully formed and a length of vanishing threads. The complementary coupling member having thread machined on the internal surface thereof exhibits a full height tapered thread throughout the entire length thereof. The complementary threads on each member have following flanks in bearing relationship and substantially normal to the longitudinal axis of the joint and leading flanks in bearing relationship, which leading flanks have a larger flank angle than the following flanks. Crests and roots are truncated to provide flat crests and roots which are parallel to the longitudinal axis of the joint. Crest truncations of fully formed threads exceed root truncations to provide a predetermined amount of void between the crests and roots of the complementary threads throughout the length of the joint when the pipe and coupling are in hand tight engagement. After power make-up, the crests of the coupling threads engage the roots of the vanishing pipe threads, but voids still remain between crests and roots of the fully formed threads, thus preventing the development of deleterious hoop stresses at the end of the coupling during power make-up.
U.S. Pat. No. 3,989,284, issued Nov. 2, 1976 to Thomas L. Blose (see also U.S. Pat. No. Re. 30,647, issued Jun. 16, 1981) describes a tubular connection (pipe joint) designed to produce hoop tension in the pin member and hoop compression in the box member, the connection exhibits threads having a dove-tail interfit. To achieve the dove-tail interfit, the angles formed between the thread flanks and the root wall of the thread are less than about 85.degree.. In another described embodiment, the load flank of the thread exhibits an angularity from the direction of the connection on axis of less than about 85.degree. while with the stab flank extends radially, in axial radial planes. The pin thread stab flank is the thread flank which is on the leading edge as the pin is telescoped (stabbed) into the box. The load flank is the trailing flank of the thread. In all cases, the threads are designed with clearances such that there is a "dove-tail interfit" between pin and box drawing the pin wall outwardly and the box wall inwardly to produce hoop tension in the pin member and hoop compression in the box member in a made up connection, i.e. there is a wedging interfit of threads in the made up joint.
Each thread member is disclosed in the '284 Blose patent as having progressively changing axial width along substantially the entire helical length thereof. Although the preferred embodiment appears to have the threads constructed on a line parallel to the pipe longitudinal axis, the possibility is disclosed of having the threads constructed on a line having constant taper from the pipe longitudinal axis. Also described is the use of an inner shoulder seal between threaded sections of the connection. In one embodiment, the inner shoulder seal is placed between two thread sections which fall on two different lines, each parallel to the pipe longitudinal axis, creating a multiple-step thread with an inner shoulder seal. The inner shoulder seal is shown and described as having a major mating (sealing) surface which is parallel to the pipe longitudinal axis or a major mating (sealing) surface which is at an axial taper.
U.S. Pat. No. 4,009,893, issued Mar. 1, 1977 to Schatton et al., discloses box and pin members having two axially spaced thread sections being separated by a radial step zone which includes, for one member, eg. the pin member, an annulus having an axial undercut, a frusto-conical end face and a radially outward bulging peripheral surface, which sealingly engages a slightly contoured surface of the box member in the step zone, while the end face of the annulus bears against a complementary, slightly contoured face in the step zone of the box member. These stop faces supplement complementary thread stop faces respectively at the end of the pin member and box member. It is advised that a flat annulus having good sliding properties, such as teflon, be interposed between the pin member annulus end face and the complementary bearing face of the box member. In one embodiment, there is a space between the pin member annulus end face and the complementary beating face of the box member, and a sealing sleeve is interposed to fill this space and the undercut space of the pin member annulus.
U.S. Pat. No. 4,076,436 to D. T. Slatoret al., issued Feb. 28, 1979 describes specially designed stress relief grooves for threaded end portions of tools and for collars on reciprocating or rotating drill string mounted tools.
U.S. Pat. No. 4,161,332, issued Jul. 17, 1979 to Thomas L. Blose describes a pipe joint including pin and box members having interengaged two-step threads and interengaged annular shoulders. One of the annular shoulders of the connection is located axially between the first and second pair of threads. This shoulder is constructed so that the pin member stab flank of the shoulder is at a negative angle to the pipe longitudinal axis (preferably at about 5 degrees) to provide a semi-dovetail interfit of the shoulder on makeup of the joint. The pin member threads in general exhibit a negative angle load flank (preferably about 15 degrees) and a negative stab flank angle (typically about 30 degrees), to provide a semi-dovetail interfit with corresponding box threads; the pin member stab flank is typically not engaged, there being about 0.020 inches clearance with the corresponding box threads on normal make-up.
U.S. Pat. No. 4,224,607 to T. L. Blose, issued Jul. 17, 1979, discloses a cylindrical threaded connector having a small amount of taper to produce a controlled radial interference to prevent inadvertent thread disengagement.
U.S. Pat. No. 4,373,754 to C. A. Bollfrass, et at., issued Feb. 15, 1983, describes a threaded connector having multi-stepped, non-tapered thread sections. The thread profile has at least one "hooked" flank to provide improved tensile strength. The wall thickness of the female or box member of the connector is controlled to improve thread loading and to enable inwardly radial movement with the male or pin member to prevent "pulling out" of the threads under axial tensile loading. The outer thread step of the pin member may only partially engage the threads of the box member such that the critical reduction of the tube wall thickness at the thread roots is a minimum.
U.S. Pat. No. 4,384,737 to H. E. Reusser, issued May 24, 1983 discloses a tubular connection including a coupling or integral socket into which pipe sections are threaded. The outer surface of the pipe sections includes an unthreaded nose portion at the outer end of the threaded portion. The nose portion terminates with an annular surface and includes a plurality of convex surfaces of revolution extending circumferentially of the nose portion, all of which surfaces are adapted to engage the unthreaded portion of the coupling or socket to establish a fluid-tight seal. The present invention does not utilize such a pin nose design. Another feature of the Reusser invention (which is present in all claims which do not pertain specifically to the above-described pin nose design) is that surface-to-surface thread contact exists between either the minor diameter flat portions of thread on each member or between the major diameter flat portions of thread on each member, but that there not be contact between both the minor and major diameter flat portions. The present invention does not use this method of controlling stresses created during make-up of the connection. The connection of the present invention exhibits surface-to-surface thread contact between the crests and roots of successive threads for all full height threads and the majority (of not all) partial height threads.
U.S. Pat. No. 4,398,756, issued Aug. 16, 1983 to Duret et al., describes a cylindro-conical pipe joint wherein approximately 15% to 25% of the joint is threaded upon a continuous construction line which is at a taper to the longitudinal axis of the pipe, with the remaining approximately 75% to 85% of the threaded portion of the joint being threaded upon a continuous construction line parallel to the longitudinal axis of the pipe. The position of the threading which is on a taper to the pipe longitudinal axis is that which forms the entry to the connection when the pin and box are initially being telescoped together. Thus, clearance is provided by the taper of the box entry to permit ease in initial telescoping of the pin into the box, prior to actual engagement of the threads. Another feature of the Duret et al. invention is that the female (box) member has an internal shoulder against which the end face of the male (pin) member seals. The connection of the present invention relies on other sealing surfaces within the central structure of the connection, permitting the end face of the pin member to extend openly at the edge of the connection.
U.S. Pat. No. 4,570,982 to T. L. Blose, issued Feb. 18, 1986, describes a tubular connection including pin and box members having a two step thread profile with the leading thread step of the pin member being a smaller diameter than the second thread step. The thread profile of the members has a negative angle load flank and the thread steps are axially separated from one another by a reverse angle torque shoulder having one or two adjacent conical surfaces. The negative load flank angle of the threads and the reverse shoulder angle combine to lock the members together and prevent differential radial separation of the engaged conical sealing surfaces.
U.S. Pat. No. 4,591,195 issued May 27, 1986 to Chelette et al. discloses a center shoulder seal located between two threaded portions of a pipe joint, wherein the threaded portions are stepped threads constructed on a line parallel to the longitudinal axis of the pipe. The center shoulder seal is fabricated so the pin portion of the seal contains an undercut groove which can be used in combination with a sealing ring of rigid plastic or rubber material to provide a seal capable of retaining hot gases or liquids.
U.S. Pat. No. 4,603,889 to J. W. Welsh, issued Aug. 8, 1986, discloses a threaded fastener having an axis adapted to thread connect to two bodies. The fastener is threaded on the inside and the two bodies are threaded on the outside. The threading has first and second load flanks which face axially oppositely. The first flank defines a second pitch, said pitches being different and characterized in that when the fastener is screwed onto body threads the first flank tightens against one body thread when the second flank tightens against the other body thread.
U.S. Pat. No. 4,671,544, issued Jun. 9, 1987 to Donald J. Ortloff discloses a threaded pipe connection having two portions of tapered threads with tapered sealing surfaces located between the threaded portions. The tapered sealing surfaces engage a resilient seal member located in a groove about midway between the ends of one of the seal member surfaces; when the joint is made up, the sealing surfaces form independent metal-to-metal seals on both sides of the resilient seal member. In a preferred embodiment, the threaded portions are in the form of two steps, one on each side of the center sealing surface described above; the threads are wedge-shaped threads that increase in width progressively in one direction within each step of threads.
U.S. Pat. No. 4,676,529, issued Jun. 30, 1987 to Patrick E. McDonald describes a pipe joint wherein the pin and box members have two stepped portions of threads, each constructed on a line parallel with the longitudinal axis of the pipe, and having metal-to-metal sealing surfaces located between the threaded portions. The sealing surfaces provide three engaging shoulders between the pin and box; the two sets of outer shoulders have space between them at the time the set of inner shoulders is in contact. Between the set of inner shoulders and each set of outer shoulders, there is a groove; in one location the groove is in the box portion of the connection and in the other location the groove is in the pin portion of the connection. When the connection make-up torque is beyond the yield point of the metal of the set of inner shoulders, the metal extrudes into the grooves in the box and pin members to form interlocking surfaces which hold the box and pin from relative rotation.
U.S. Pat. No. 4,696,498 to K. L. Church, issued Sep. 29, 1987, describes a tubular connection having two members, one threaded on the inside (box) and one threaded on the outside (pin). Each member has a double set of steep tapered hook threads, two internal pressure seals including a compressible primary metal-to-metal seal and a supplementary seal made up of a resilient corrosion resistant material. Additionally, the connection has a primary resilient corrosion resistant external pressure seal, and a reverse shoulder engagement between the end of the pin member and a shoulder on the box member to hold the primary metal sealing surfaces in sealing engagement.
U.S. Pat. No. 4,703,954, issued Nov. 3, 1987 to Ortloff et al. describes a threaded pipe connection comprising a box having tapered internal threads and a pin having tapered external threads, wherein the threads are dovetail shaped in cross section (wedge shaped thread) and increase in width in one direction on the box and in the other direction on the pin. This design is improved over that described in U.S. Pat. No. 3,989,284 (U.S. Pat. No. Re. 30,647) in that high stress concentrations are avoided in the box between the first thread and the end of the box by making the length of the box between the first thread and the end of the box equal to or greater than 5% of the pipe diameter and by providing clearance-between the crest of the first full thread on the box and the root of the last thread on the pin and between the crest of the last full thread on the pin and the root of the first full thread on the box.
U.S. Pat. No. 4,703,959 to E. E. Reeves et al., issued Nov. 3, 1987 discloses a threaded pipe connection with compressible seal ring. The connection comprises a pin member threaded on the outside and a box member threaded on the inside. The members have threading which is tapered, with generally dovetail-shaped threads that increase in width in one direction on the pin and in the opposite direction on the box so that the pin and box threads engage to form thread seals between the flanks of the threads and between the roots and crests of the threads. The improvement is said to comprise an annular groove intermediate the ends of the threads on one of the box and pin to form a cavity between the groove and the threads on the other of the box and pin and a ring-shaped seal member of compressible material located in the annular groove.
U.S. Pat. No. 4,753,460, issued Jun. 28, 1988 to Lawrence Y. Tung, discloses a tubular connection having a small diameter thread set and a large diameter thread set, the small thread set having a lesser number of threads than the large set. A primary torque shoulder is located between the thread sets and a secondary torque shoulder is located behind the large thread set. In the preferred embodiment, the initial partial thread on the pin of both sets and the last partial thread of the small set on the box has a cylindrical root contour (the root contour is constructed on a line parallel with the longitudinal axis of the pipe), whereas the remainder of threads exhibit a tapered root contour. The longitudinal taper angle of the pin thread sets are dissimilar to the box thread sets, there being a maximum separation of the sets near the center of the connection. At the center, a seal is provided; the last threads adjacent to the center shoulder on the threaded sets are hooked (the angle of the pin load flank is negative with respect to the pipe longitudinal axis). In all embodiments described, the threads exhibit a negative load flank and a positive stab flank. The center shoulder area of the pin exhibits two flanks, one of which is positive (near the small diameter step of threads) and one of which is negative (near the center of the shoulder and about midway between the two steps of threads).
U.S. Pat. No. 4,822,081 to T. L. Blose, issued Apr. 18, 1989, describes a driveable two-member tubular connection, and in particular a thread form for the connection. The thread form utilizes converging, trapped thread flanks and mating shoulders and end faces with dissimilar angles. A two step thread design is illustrated, wherein variable width "trapped" threads are present on each step of threads. The thread crests and roots are described as making contact before the thread flanks are touching. This Blose et al. invention is concerned with the benefit of the thread stab flanks in assisting the shoulders/end faces in resisting axial compressive loading and with the different angles between mating shoulders and end faces to reduce deformation of the shoulders upon axial compressive loading.
U.S. Pat. No. 4,893,844 to K. D. Chelette et al., issued Jan. 16, 1990, discloses a tubular connection with ventable seal. A deforming seal member is disposed between two surfaces such that a space is provided to at least one side of the sealing member. The connection comprises box and pin members having hooked threads, a stop shoulder between the two members and various seal means.
U.S. Pat. No. 4,917,409, issued Apr. 17, 1990 to Doyle E. Reeves describes a threaded connection employing a tapered, wedge shaped thread. The threads increase in width in one direction on the box and in the other direction on the pin. The improvement over the prior art appears to be in the ability of the thread design to be used with pipe thread lubricants; to accommodate such lubricants, the thread is designed to eliminate the radial clearance between the thread roots and crests so that the possibility of creating an expansible chamber upon connection make-up that can entrap the liquid or paste-like thread lubricant therein, to produce a false torque reading is greatly minimized, if not entirely reduced. The wedge-shaped thread can be a dovetail or semi-dovetail thread profile. A variable width thread construction (wherein thread width increases progressively in a given direction) is used in combination with the dovetail or semi-dovetail thread profile (as in the Blose U.S. Pat. No. 3,989,284). The specification and original claims of Reeves discloses that the load and stab flanks of the thread fully engage "at substantially the same time upon rotational make-up of the threaded connection" Col. 4, lines 43-48. Although only continuation applications were filed during prosecution (no continuation-in-part applications which would permit the introduction of new matter), the claims were amended in a subsequent continuation application to contain the limitation that the thread flanks did not fully engage at substantially the same time. Thus, there is presently some confusion as to exactly what is taught by Reeves.
Another concept disclosed and dependently claimed in Reeves is that it is desirable to increase the thread width at the widest portion in the range of approximately four times the thread width at the narrowest portion at the same thread location. Them is no discussion as to why this provides an advantage over prior art (which is said to show less of an increase in thread width).
U.S. Pat. No. 4,944,538 to S. R. Read, issued Jul. 31, 1990, describes a threaded pipe joint having improved seal ring entrapment.
U.S. Pat. No. 4,958,862 to C. Capelli et al., issued Sep. 25, 1990, discloses a tubular connection comprising a pin member having at least one external tapered thread and a box member with complementary internal threads. An end part of the pin member beyond each thread has a conical seal surface and the box member has a conical seal surface with the same taper as that of the pin member, so that upon tightening of the joint a seal is created between pin and box at the forward end region of the pin member.
U.S. Pat. No. 5,029,906 to K. D. Chelette et al., issued Jul. 9, 1991, pertains to a deformable sealing member used to provide a pressure vent for mating tubular members. A typical threaded coupling comprises pin and box members each having a stepped thread design of hooked configuration on at least one side of a stop shoulder.
U.K. Patent Specification 137,777 of W. Pickard, published in 1920, shows a variety of thread designs which can be used to connect two or more parts or objects laterally or which, on entering a material, will become anchored therein and resist separation in all directions at right angles to the longitudinal axis of the connection. FIG. 8 of the disclosure shows a single hook thread profile. FIG. 10 shows a double hook thread profile.
U.K. Patent Application of K. Maruyama, published Nov. 4, 1981, describes a fluid tight thread joint for oil or gas well pipes. The joint comprises box and pin members; the thread on the pin starts from the pin end as a rounded triangular thread and becomes a buttress thread; the thread on the box member is formed in a corresponding manner.